Devices have been known for a long while for connecting together two machine parts formed with rotationally generated surfaces shaped alike, said surfaces being coaxial at the coupling, and, by utilizing hydraulic pressure, said devices bring about expansion of one of the rotationally generated surfaces to give coupling contact with the other one. Examples on such known devices are revealed in U.S. Pat. No. 4,093,052 and French Pat. No. 1,092,416. The hydraulic systems used up to now for this purpose contain an annular working chamber arranged in side one of the rotationally generated surfaces, said chamber having a wall thickness towards the coupling surface such that this wall, when the specific pressure on the hydraulic medium enclosed in the chamber is increased, elastically expands to give the desired coupling connection. The working chamber in the known devices has been uniformly thick, with relatively large space (in the order of millimeters) between its coaxial inner defining surfaces in relation to the space or clearance (some hundreths of a millimeter) between the rotationally generated surfaces which are to be coupled together. Such a construction has in many cases been found to give unsatisfactory mechanical stability to the device, e.g. in the cases where the load on the expanding wall varies along the longitudinal direction thereof.